The present invention relates to a composition for injection molding and a composition for vehicular exterior material. More particularly, the present invention is concerned with a composition for injection molding and a composition for vehicular exterior material both containing a partially crosslinked product prepared by dynamically heat-treating an .alpha.-olefin (co)polymer having 2 to 8 carbon atoms and a rubbery substance as well as, if necessary, an inorganic filler and/or a softening agent in the presence of a specific crosslinking agent and a polyfunctional monomer. The compositions of the present invention are well-balanced in rigidity, impact resistance, appearance of molded products and moldability, and are employable for baskets and bamboo baskets for documents sorting, kitchen use, dressing room use, fruits and refuse, for lids of various things of daily necessities, namely, for caps of wide-mouth bottles, toy parts, and also for vehicular exterior materials such as rear finisher, sill, rear coater panel, engine hood, trunk lid, fender, door panel, protector, bumper fascia, energy absorber, air spoiler, side molding, weather strip, shock absorber, dust boots, vacuum connector, rack and pinion boots, and mud guard.
Thermoplastic elastomer (TPE) is mentioned as one of those commonly used heretofore as thermoplastic resins for injection molding. Above all, olefinic thermoplastic elastomers are used frequently because they are inexpensive.
Generally, such olefinic thermoplastic elastomers are blends each comprising as a hard segment a crystalline polypropylene (hereinafter referred to simply as "PP") or polyethylene and as a soft segment an ethylene-.alpha.-olefin copolymer rubber, e.g. ethylene-propylene rubber (hereinafter referred to simply as "EPR") or ethylene-propylene-diene random copolymer rubber, or an olefinic elastomer such as polyisobutylene or polybutadiene. Partially crosslinking is often conducted for the improvement of physical properties.
For example, in Japanese Patent Laid-Open No. 41950/1979 there is disclosed a composition prepared by kneading PP and EPR with polyethylene having a density of 0.915 to 0.97 and peroxide. Further, in Japanese Patent Laid-Open No. 139445/1980 there is proposed a composition comprising PP, EPR, a high-density polyethylene and talc, and in Japanese Patent Laid-Open Nos. 42643/1983 and 80335/1983 there is proposed a composition comprising PP, EPR and a linear ethylene-.alpha.-olefin copolymer.
The conventional processes for the preparation of such injection molding compositions are broadly classified into the following two.
The first process is a mere blending of a thermoplastic resin with a rubbery substance. This process involves problems such as deterioration of mechanical characteristics and of heat resistance, ineffective improvement of impact resistance, and poor appearance of molded products such as the presence of weld mark and flow mark.
According to the second process, an organic peroxide is added at the time of kneading of a thermoplastic resin and a rubbery substance to enhance the effect of improving mechanical characteristics as compared with that obtained in the aforementioned simple blending. Thus, an organic peroxide is used in this process, so when a peroxide degradation type resin such as polypropylene is used as the hard segment, there rapidly occurs thermal decomposition of the organic peroxide, with the result that a molecular cutting reaction is unavoidable, thus leading to deterioration in mechanical characteristics such as flexural modulus and yield tensile strength.
In the case of using a peroxide crosslinking type resin such as polyethylene, a crosslinking reaction of the hard segment proceeds and fluidity is deteriorated markedly, resulting in that the moldability is deteriorated to the extent of molding being difficult, and in order to ensure good moldability it is necessary to use an extremely small amount of an organic peroxide. Thus, the handling is difficult.
Anyhow, even in a crosslinking reaction using a peroxide, deterioration of mechanical characteristics is unavoidable as in the foregoing simple blend; besides, the surface appearance of a molded product obtained is inferior, such as the presence of weld mark or flow mark.
Recently, the use of TPO for injection molding has been expanded to the automobile industry, i.e., automobile parts, and at the same time performance conditions required have been more and more diversified.
For example, as to automobile exterior materials, not only high heat resistance and low-temperature impact resistance but also good moldability, product appearance and coatability are required. It is difficult for conventional thermoplastic elastomers (TPE) to satisfy all of these requirements. Particularly, with conventional TPE's, the balance between low-temperature impact resistance and the surface appearance of molded products, such as whether weld mark or flow mark is present or not, is still unsatisfactory.
Particularly, where the inorganic filler is a metal oxide (or hydroxide), the receptivity of polyolefin resins is low and the dispersibility is poor, so molded products obtained are inferior not only in the appearance thereof but also in the mechanical characteristics thereof. To avoid this, it is necessary to add a soft substance or a high-molecular compound having a polar group.
Thus, the thermoplastic resin compositions for injection molding prepared by the above conventional processes are not fully satisfactory in practical use, so the provision of an improved and superior composition is desired.
The present invention has been accomplished in view of the above-mentioned points and it is an object thereof to provide a composition for injection molding well balanced in mechanical characteristics, appearance of a molded product obtained using the composition and moldability, and having improved compatibility with thermoplastic resin compositions and inorganic fillers, by using a partially crosslinked product prepared by dynamically heat-treating an .alpha.-olefin (co)polymer and a rubbery substance in the presence of a special crosslinking agent and a polyfunctional monomer, and, if necessary, also using an inorganic filler and/or a softening agent, all in specific proportions.
It is another object of the present invention to provide a composition for vehicular exterior material of high performance superior in low-temperature impact resistance and having a beautiful surface appearance, which composition is constituted by a thermoplastic resin composition containing a partially crosslinked product obtained by dynamically heat-treating a specific amount of a polypropylene resin, or a polyethylene resin if desired, and a rubbery substance in the presence of a special crosslinking agent and a polyfunctional monomer, or such thermoplastic resin further containing a softening agent and/or an inorganic filler.